W3 is a region where many massive stars are forming in a string of stellar
clusters, located about 6,000 light years from Earth in the Perseus arm of
the Milky Way galaxy. W3 is part of a vast molecular cloud complex that
also contains the W4 superbubble (not seen in this image). Scientists
believe that the extraordinary amount of star formation in W3 has possibly
been influenced by neighboring W4, an inflating bubble of gas over 100
light years across. W4 may directly trigger the birth of W3's massive
stellar clusters as it expands and sweeps up molecular gas into a
high-density layer at its edge, within which stars can form. Another
possible scenario is that W4's expansion has caused a domino effect of star
formation, forming the cluster IC 1795 (seen as a clump of X-ray sources in
the bottom left corner of this image) which in turn triggered formation of
the young, massive clusters in W3.

In this composite image of one of the many star-forming complexes of W3,
called W3 Main, green and blue represent lower and higher-energy X-rays,
respectively, while red shows optical emission. Hundreds of X-ray sources
are revealed in this central region of W3 Main. These bright point-like
objects are an extensive population of several hundred young stars, many of
which were not found in earlier infrared studies. These Chandra data show
that W3 Main is the dominant star formation region of W3. Because its
X-ray sources are all at the same distance, yet span a range of masses,
ages, and other properties, W3 is an ideal laboratory for understanding
recent and ongoing star formation in one of the Milky Way's spiral arms.